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RFC 3435

 
 
 

Media Gateway Control Protocol (MGCP) Version 1.0

Part 2 of 8, p. 10 to 32
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2. Media Gateway Control Interface

   The interface functions provide for connection control and endpoint
   control.  Both use the same system model and the same naming
   conventions.

2.1 Model and Naming Conventions

   The MGCP assumes a connection model where the basic constructs are
   endpoints and connections.  Connections are grouped in calls.  One or
   more connections can belong to one call.  Connections and calls are
   set up at the initiative of one or more Call Agents.

2.1.1 Types of Endpoints

   In the introduction, we presented several classes of gateways.  Such
   classifications, however, can be misleading.  Manufacturers can
   arbitrarily decide to provide several types of services in a single
   package.  A single product could well, for example, provide some
   trunk connections to telephony switches, some primary rate
   connections and some analog line interfaces, thus sharing the
   characteristics of what we described in the introduction as
   "trunking", "access" and "residential" gateways.  MGCP does not make
   assumptions about such groupings.  We simply assume that media
   gateways support collections of endpoints.  The type of the endpoint
   determines its functionality.  Our analysis, so far, has led us to
   isolate the following basic endpoint types:

   * Digital channel (DS0),

   * Analog line,

   * Announcement server access point,

   * Interactive Voice Response access point,

   * Conference bridge access point,

   * Packet relay,

   * ATM "trunk side" interface.

   In this section, we will describe the expected behavior of such
   endpoints.

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   This list is not final.  There may be other types of endpoints
   defined in the future, for example test endpoints that could be used
   to check network quality, or frame-relay endpoints that could be used
   to manage audio channels multiplexed over a frame-relay virtual
   circuit.

2.1.1.1 Digital Channel (DS0)

   Digital channels provide a 64 Kbps service.  Such channels are found
   in trunk and ISDN interfaces.  They are typically part of digital
   multiplexes, such as T1, E1, T3 or E3 interfaces.  Media gateways
   that support such channels are capable of translating the digital
   signals received on the channel, which may be encoded according to
   A-law or mu-law, using either the complete set of 8 bits per sample
   or only 7 of these bits, into audio packets.  When the media gateway
   also supports a Network Access Server (NAS) service, the gateway
   shall be capable of receiving either audio-encoded data (modem
   connection) or binary data (ISDN connection) and convert them into
   data packets.

                                         +-------
                           +------------+|
              (channel) ===|DS0 endpoint| -------- Connections
                           +------------+|
                                         +-------

   Media gateways should be able to establish several connections
   between the endpoint and the packet networks, or between the endpoint
   and other endpoints in the same gateway.  The signals originating
   from these connections shall be mixed according to the connection
   "mode", as specified later in this document.  The precise number of
   connections that an endpoint supports is a characteristic of the
   gateway, and may in fact vary according to the allocation of
   resources within the gateway.

   In some cases, digital channels are used to carry signaling.  This is
   the case for example for SS7 "F" links, or ISDN "D" channels.  Media
   gateways that support these signaling functions shall be able to send
   and receive the signaling packets to and from a Call Agent, using the
   "backhaul" procedures defined by the SIGTRAN working group of the
   IETF.  Digital channels are sometimes used in conjunction with
   channel associated signaling, such as "MF R2".  Media gateways that
   support these signaling functions shall be able to detect and produce
   the corresponding signals, such as for example "wink" or "A",
   according to the event signaling and reporting procedures defined in
   MGCP.

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2.1.1.2 Analog Line

   Analog lines can be used either as a "client" interface, providing
   service to a classic telephone unit, or as a "service" interface,
   allowing the gateway to send and receive analog calls.  When the
   media gateway also supports a NAS service, the gateway shall be
   capable of receiving audio-encoded data (modem connection) and
   convert them into data packets.

                                         +-------
                        +---------------+|
              (line) ===|analog endpoint| -------- Connections
                        +---------------+|
                                         +-------

   Media gateways should be able to establish several connections
   between the endpoint and the packet networks, or between the endpoint
   and other endpoints in the same gateway.  The audio signals
   originating from these connections shall be mixed according to the
   connection "mode", as specified later in this document.  The precise
   number of connections that an endpoint supports is a characteristic
   of the gateway, and may in fact vary according to the allocation of
   resources within the gateway.  A typical gateway should however be
   able to support two or three connections per endpoint, in order to
   support services such as "call waiting" or "three way calling".

2.1.1.3 Announcement Server Access Point

   An announcement server endpoint provides access to an announcement
   service.  Under requests from the Call Agent, the announcement server
   will "play" a specified announcement.  The requests from the Call
   Agent will follow the event signaling and reporting procedures
   defined in MGCP.

                  +----------------------+
                  | Announcement endpoint| -------- Connection
                  +----------------------+

   A given announcement endpoint is not expected to support more than
   one connection at a time.  If several connections were established to
   the same endpoint, then the same announcements would be played
   simultaneously over all the connections.

   Connections to an announcement server are typically one way, or "half
   duplex" -- the announcement server is not expected to listen to the
   audio signals from the connection.

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2.1.1.4 Interactive Voice Response Access Point

   An Interactive Voice Response (IVR) endpoint provides access to an
   IVR service.  Under requests from the Call Agent, the IVR server will
   "play" announcements and tones, and will "listen" to responses, such
   as DTMF input or voice messages, from the user.  The requests from
   the Call Agent will follow the event signaling and reporting
   procedures defined in MGCP.

                      +-------------+
                      | IVR endpoint| -------- Connection
                      +-------------+

   A given IVR endpoint is not expected to support more than one
   connection at a time.  If several connections were established to the
   same endpoint, then the same tones and announcements would be played
   simultaneously over all the connections.

2.1.1.5 Conference Bridge Access Point

   A conference bridge endpoint is used to provide access to a specific
   conference.

                                           +-------
               +--------------------------+|
               |Conference bridge endpoint| -------- Connections
               +--------------------------+|
                                           +-------

   Media gateways should be able to establish several connections
   between the endpoint and the packet networks, or between the endpoint
   and other endpoints in the same gateway.  The signals originating
   from these connections shall be mixed according to the connection
   "mode", as specified later in this document.  The precise number of
   connections that an endpoint supports is a characteristic of the
   gateway, and may in fact vary according to the allocation of
   resources within the gateway.

2.1.1.6 Packet Relay

   A packet relay endpoint is a specific form of conference bridge, that
   typically only supports two connections.  Packets relays can be found
   in firewalls between a protected and an open network, or in
   transcoding servers used to provide interoperation between
   incompatible gateways, for example gateways that do not support
   compatible compression algorithms, or gateways that operate over
   different transmission networks such as IP and ATM.

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                                           +-------
                   +---------------------+ |
                   |Packet relay endpoint|  2 connections
                   +---------------------+ |
                                           +-------

2.1.1.7 ATM "trunk side" Interface

   ATM "trunk side" endpoints are typically found when one or several
   ATM permanent virtual circuits are used as a replacement for the
   classic "TDM" trunks linking switches.  When ATM/AAL2 is used,
   several trunks or channels are multiplexed on a single virtual
   circuit; each of these trunks correspond to a single endpoint.

                                          +-------
                      +------------------+|
          (channel) = |ATM trunk endpoint| -------- Connections
                      +------------------+|
                                          +-------

   Media gateways should be able to establish several connections
   between the endpoint and the packet networks, or between the endpoint
   and other endpoints in the same gateway.  The signals originating
   from these connections shall be mixed according to the connection
   "mode", as specified later in this document.  The precise number of
   connections that an endpoint supports is a characteristic of the
   gateway, and may in fact vary according to the allocation of
   resources within the gateway.

2.1.2 Endpoint Identifiers

   Endpoint identifiers have two components that both are case-
   insensitive:

   * the domain name of the gateway that is managing the endpoint

   * a local name within that gateway

   Endpoint names are of the form:

      local-endpoint-name@domain-name

   where domain-name is an absolute domain-name as defined in RFC 1034
   and includes a host portion, thus an example domain-name could be:

      mygateway.whatever.net

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   Also, domain-name may be an IP-address of the form defined for domain
   name in RFC 821, thus another example could be (see RFC 821 for
   details):

      [192.168.1.2]

   Both IPv4 and IPv6 addresses can be specified, however use of IP
   addresses as endpoint identifiers is generally discouraged.

   Note that since the domain name portion is part of the endpoint
   identifier, different forms or different values referring to the same
   entity are not freely interchangeable.  The most recently supplied
   form and value MUST always be used.

   The local endpoint name is case-insensitive.  The syntax of the local
   endpoint name is hierarchical, where the least specific component of
   the name is the leftmost term, and the most specific component is the
   rightmost term.  The precise syntax depends on the type of endpoint
   being named and MAY start with a term that identifies the endpoint
   type.  In any case, the local endpoint name MUST adhere to the
   following naming rules:

   1) The individual terms of the naming path MUST be separated by a
      single slash ("/", ASCII 2F hex).

   2) The individual terms are character strings composed of letters,
      digits or other printable characters, with the exception of
      characters used as delimiters ("/", "@"), characters used for
      wildcarding ("*", "$") and white spaces.

   3) Wild-carding is represented either by an asterisk ("*") or a
      dollar sign ("$") for the terms of the naming path which are to be
      wild-carded.  Thus, if the full local endpoint name is of the
      form:

          term1/term2/term3

      then the entity name field looks like this depending on which
      terms are wild-carded:

          */term2/term3 if term1 is wild-carded
          term1/*/term3 if term2 is wild-carded
          term1/term2/* if term3 is wild-carded
          term1/*/*     if term2 and term3 are wild-carded, etc.

      In each of these examples a dollar sign could have appeared
      instead of an asterisk.

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   4) A term represented by an asterisk ("*") is to be interpreted as:
      "use ALL values of this term known within the scope of the Media
      Gateway".  Unless specified otherwise, this refers to all
      endpoints configured for service, regardless of their actual
      service state, i.e., in-service or out-of-service.

   5) A term represented by a dollar sign ("$") is to be interpreted as:
      "use ANY ONE value of this term known within the scope of the
      Media Gateway".  Unless specified otherwise, this only refers to
      endpoints that are in-service.

   Furthermore, it is RECOMMENDED that Call Agents adhere to the
   following:

   * Wild-carding should only be done from the right, thus if a term is
     wild-carded, then all terms to the right of that term should be
     wild-carded as well.

   * In cases where mixed dollar sign and asterisk wild-cards are used,
     dollar-signs should only be used from the right, thus if a term had
     a dollar sign wild-card, all terms to the right of that term should
     also contain dollar sign wild-cards.

   The description of a specific command may add further criteria for
   selection within the general rules given above.

   Note, that wild-cards may be applied to more than one term in which
   case they shall be evaluated from left to right.  For example, if we
   have the endpoint names "a/1", "a/2", "b/1", and "b/2", then "$/*"
   (which is not recommended) will evaluate to either "a/1, a/2", or
   "b/1, b/2".  However, "*/$" may evaluate to "a/1, b/1", "a/1, b/2",
   "a/2, b/1", or "a/2, b/2".  The use of mixed wild-cards in a command
   is considered error prone and is consequently discouraged.

   A local name that is composed of only a wildcard character refers to
   either all (*) or any ($) endpoints within the media gateway.

2.1.3 Calls and Connections

   Connections are created on the Call Agent on each endpoint that will
   be involved in the "call".  In the classic example of a connection
   between two "DS0" endpoints (EP1 and EP2), the Call Agents
   controlling the endpoints will establish two connections (C1 and C2):

                  +---+                            +---+
    (channel1) ===|EP1|--(C1)--...        ...(C2)--|EP2|===(channel2)
                  +---+                            +---+

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   Each connection will be designated locally by an endpoint unique
   connection identifier, and will be characterized by connection
   attributes.

   When the two endpoints are located on gateways that are managed by
   the same Call Agent, the creation is done via the three following
   steps:

   1) The Call Agent asks the first gateway to "create a connection" on
      the first endpoint.  The gateway allocates resources to that
      connection, and responds to the command by providing a "session
      description".  The session description contains the information
      necessary for a third party to send packets towards the newly
      created connection, such as for example IP address, UDP port, and
      codec parameters.

   2) The Call Agent then asks the second gateway to "create a
      connection" on the second endpoint.  The command carries the
      "session description" provided by the first gateway.  The gateway
      allocates resources to that connection, and responds to the
      command by providing its own "session description".

   3) The Call Agent then uses a "modify connection" command to provide
      this second "session description" to the first endpoint.  Once
      this is done, communication can proceed in both directions.

   When the two endpoints are located on gateways that are managed by
   two different Call Agents, the Call Agents exchange information
   through a Call-Agent to Call-Agent signaling protocol, e.g., SIP [7],
   in order to synchronize the creation of the connection on the two
   endpoints.

   Once a connection has been established, the connection parameters can
   be modified at any time by a "modify connection" command.  The Call
   Agent may for example instruct the gateway to change the codec used
   on a connection, or to modify the IP address and UDP port to which
   data should be sent, if a connection is "redirected".

   The Call Agent removes a connection by sending a "delete connection"
   command to the gateway.  The gateway may also, under some
   circumstances, inform a gateway that a connection could not be
   sustained.

   The following diagram provides a view of the states of a connection,
   as seen from the gateway:

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           Create connection
              received
                  |
                  V
         +-------------------+
         |resource allocation|-(failed)-+
         +-------------------+          |
                  |           (connection refused)
            (successful)
                  |
                  v
     +----------->+
     |            |
     |   +-------------------+
     |   |  remote session   |
     |   |   description     |----------(yes)--------+
     |   |    available ?    |                       |
     |   +-------------------+                       |
     |            |                                  |
     |          (no)                                 |
     |            |                                  |
     |      +-----------+                         +------+
     | +--->| half open |------> Delete   <-------| open |<----------+
     | |    |  (wait)   |      Connection         |(wait)|           |
     | |    +-----------+       received          +------+           |
     | |          |                 |                |               |
     | |   Modify Connection        |         Modify Connection      |
     | |      received              |            received            |
     | |          |                 |                |               |
     | | +--------------------+     |       +--------------------+   |
     | | |assess modification |     |       |assess modification |   |
     | | +--------------------+     |       +--------------------+   |
     | |    |             |         |          |             |       |
     | |(failed)     (successful)   |      (failed)     (successful) |
     | |    |             |         |          |             |       |
     | +<---+             |         |          +-------------+-------+
     |                    |         |
     +<-------------------+         |
                                    |
                           +-----------------+
                           | Free connection |
                           | resources.      |
                           | Report.         |
                           +-----------------+
                                    |
                                    V

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2.1.3.1 Names of Calls

   One of the attributes of each connection is the "call identifier",
   which as far as the MGCP protocol is concerned has little semantic
   meaning, and is mainly retained for backwards compatibility.

   Calls are identified by unique identifiers, independent of the
   underlying platforms or agents.  Call identifiers are hexadecimal
   strings, which are created by the Call Agent.  The maximum length of
   call identifiers is 32 characters.

   Call identifiers are expected to be unique within the system, or at a
   minimum, unique within the collection of Call Agents that control the
   same gateways.  From the gateway's perspective, the Call identifier
   is thus unique.  When a Call Agent builds several connections that
   pertain to the same call, either on the same gateway or in different
   gateways, these connections that belong to the same call should share
   the same call-id.  This identifier can then be used by accounting or
   management procedures, which are outside the scope of MGCP.

2.1.3.2 Names of Connections

   Connection identifiers are created by the gateway when it is
   requested to create a connection.  They identify the connection
   within the context of an endpoint.  Connection identifiers are
   treated in MGCP as hexadecimal strings.  The gateway MUST make sure
   that a proper waiting period, at least 3 minutes, elapses between the
   end of a connection that used this identifier and its use in a new
   connection for the same endpoint (gateways MAY decide to use
   identifiers that are unique within the context of the gateway).  The
   maximum length of a connection identifier is 32 characters.

2.1.3.3 Management of Resources, Attributes of Connections

   Many types of resources will be associated to a connection, such as
   specific signal processing functions or packetization functions.
   Generally, these resources fall in two categories:

   1) Externally visible resources, that affect the format of "the bits
      on the network" and must be communicated to the second endpoint
      involved in the connection.

   2) Internal resources, that determine which signal is being sent over
      the connection and how the received signals are processed by the
      endpoint.

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   The resources allocated to a connection, and more generally the
   handling of the connection, are chosen by the gateway under
   instructions from the Call Agent.  The Call Agent will provide these
   instructions by sending two sets of parameters to the gateway:

   1) The local directives instruct the gateway on the choice of
      resources that should be used for a connection,

   2) When available, the "session description" provided by the other
      end of the connection (referred to as the remote session
      description).

   The local directives specify such parameters as the mode of the
   connection (e.g., send-only, or send-receive), preferred coding or
   packetization methods, usage of echo cancellation or silence
   suppression.  (A detailed list can be found in the specification of
   the LocalConnectionOptions parameter of the CreateConnection
   command.)  Depending on the parameter, the Call Agent MAY either
   specify a value, a range of values, or no value at all.  This allows
   various implementations to implement various levels of control, from
   a very tight control where the Call Agent specifies minute details of
   the connection handling to a very loose control where the Call Agent
   only specifies broad guidelines, such as the maximum bandwidth, and
   lets the gateway choose the detailed values subject to the
   guidelines.

   Based on the value of the local directives, the gateway will
   determine the resources to allocate to the connection.  When this is
   possible, the gateway will choose values that are in line with the
   remote session description - but there is no absolute requirement
   that the parameters be exactly the same.

   Once the resources have been allocated, the gateway will compose a
   "session description" that describes the way it intends to send and
   receive packets.  Note that the session description may in some cases
   present a range of values.  For example, if the gateway is ready to
   accept one of several compression algorithms, it can provide a list
   of these accepted algorithms.

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                 Local Directives
                (from Call Agent 1)
                        |
                        V
                 +-------------+
                 | resource    |
                 | allocation  |
                 | (gateway 1) |
                 +-------------+
                   |         |
                   V         |
                 Local       |
              Parameters     V
                   |      Session
                   |    Description               Local Directives
                   |         |                   (from Call Agent 2)
                   |         +---> Transmission----+      |
                   |                (CA to CA)     |      |
                   |                               V      V
                   |                           +-------------+
                   |                           | resource    |
                   |                           | allocation  |
                   |                           | (gateway 2) |
                   |                           +-------------+
                   |                               |      |
                   |                               |      V
                   |                               |    Local
                   |                               |  Parameters
                   |                            Session
                   |                          Description
                   |         +---- Transmission<---+
                   |         |      (CA to CA)
                   V         V
                 +-------------+
                 | modification|
                 | (gateway 1) |
                 +-------------+
                   |
                   V
                 Local
              Parameters

      -- Information flow: local directives & session descriptions --

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2.1.3.4 Special Case of Local Connections

   Large gateways include a large number of endpoints which are often of
   different types.  In some networks, we may often have to set-up
   connections between endpoints that are located within the same
   gateway.  Examples of such connections may be:

   * Connecting a call to an Interactive Voice-Response unit,

   * Connecting a call to a Conferencing unit,

   * Routing a call from one endpoint to another, something often
     described as a "hairpin" connection.

   Local connections are much simpler to establish than network
   connections.  In most cases, the connection will be established
   through some local interconnecting device, such as for example a TDM
   bus.

   When two endpoints are managed by the same gateway, it is possible to
   specify the connection in a single command that conveys the names of
   the two endpoints that will be connected.  The command is essentially
   a "Create Connection" command which includes the name of the second
   endpoint in lieu of the "remote session description".

2.1.4 Names of Call Agents and Other Entities

   The media gateway control protocol has been designed to allow the
   implementation of redundant Call Agents, for enhanced network
   reliability.  This means that there is no fixed binding between
   entities and hardware platforms or network interfaces.

   Call Agent names consist of two parts, similar to endpoint names.
   Semantically, the local portion of the name does not exhibit any
   internal structure.  An example Call Agent name is:

      ca1@ca.whatever.net

   Note that both the local part and the domain name have to be
   supplied. Nevertheless, implementations are encouraged to accept call
   agent names consisting of only the domain name.

   Reliability can be improved by using the following procedures:

   * Entities such as endpoints or Call Agents are identified by their
     domain name, not their network addresses.  Several addresses can be

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     associated with a domain name.  If a command or a response cannot
     be forwarded to one of the network addresses, implementations MUST
     retry the transmission using another address.

   * Entities MAY move to another platform.  The association between a
     logical name (domain name) and the actual platform is kept in the
     domain name service.  Call Agents and Gateways MUST keep track of
     the time-to-live of the record they read from the DNS.  They MUST
     query the DNS to refresh the information if the time to live has
     expired.

   In addition to the indirection provided by the use of domain names
   and the DNS, the concept of "notified entity" is central to
   reliability and fail-over in MGCP.  The "notified entity" for an
   endpoint is the Call Agent currently controlling that endpoint.  At
   any point in time, an endpoint has one, and only one, "notified
   entity" associated with it.  The "notified entity" determines where
   the endpoint will send commands to; when the endpoint needs to send a
   command to the Call Agent, it MUST send the command to its current
   "notified entity".  The "notified entity" however does not determine
   where commands can be received from; any Call Agent can send commands
   to the endpoint.  Please refer to Section 5 for the relevant security
   considerations.

   Upon startup, the "notified entity" MUST be set to a provisioned
   value.  Most commands sent by the Call Agent include the ability to
   explicitly name the "notified entity" through the use of a
   "NotifiedEntity" parameter.  The "notified entity" will stay the same
   until either a new "NotifiedEntity" parameter is received or the
   endpoint does a warm or cold (power-cycle) restart.

   If a "NotifiedEntity" parameter is sent with an "empty" value, the
   "notified entity" for the endpoint will be set to empty.  If the
   "notified entity" for an endpoint is empty or has not been set
   explicitly (neither by a command nor by provisioning), the "notified
   entity" will then default to the source address (i.e., IP address and
   UDP port number) of the last successful non-audit command received
   for the endpoint.  Auditing will thus not change the "notified
   entity".  Use of an empty "NotifiedEntity" parameter value is
   strongly discouraged as it is error prone and eliminates the DNS-
   based fail-over and reliability mechanisms.

2.1.5 Digit Maps

   The Call Agent can ask the gateway to collect digits dialed by the
   user.  This facility is intended to be used with residential gateways
   to collect the numbers that a user dials; it can also be used with

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   trunking gateways and access gateways alike, to collect access codes,
   credit card numbers and other numbers requested by call control
   services.

   One procedure is for the gateway to notify the Call Agent of each
   individual dialed digit, as soon as they are dialed.  However, such a
   procedure generates a large number of interactions.  It is preferable
   to accumulate the dialed numbers in a buffer, and to transmit them in
   a single message.

   The problem with this accumulation approach, however, is that it is
   hard for the gateway to predict how many numbers it needs to
   accumulate before transmission.  For example, using the phone on our
   desk, we can dial the following numbers:

        ------------------------------------------------------
       |  0                     |  Local operator             |
       |  00                    |  Long distance operator     |
       |  xxxx                  |  Local extension number     |
       |  8xxxxxxx              |  Local number               |
       |  #xxxxxxx              |  Shortcut to local number at|
       |                        |  other corporate sites      |
       |  *xx                   |  Star services              |
       |  91xxxxxxxxxx          |  Long distance number       |
       |  9011 + up to 15 digits|  International number       |
        ------------------------------------------------------

   The solution to this problem is to have the Call Agent load the
   gateway with a digit map that may correspond to the dial plan.  This
   digit map is expressed using a syntax derived from the Unix system
   command, egrep.  For example, the dial plan described above results
   in the following digit map:

      (0T|00T|[1-7]xxx|8xxxxxxx|#xxxxxxx|*xx|91xxxxxxxxxx|9011x.T)

   The formal syntax of the digit map is described by the DigitMap rule
   in the formal syntax description of the protocol (see Appendix A) -
   support for basic digit map letters is REQUIRED while support for
   extension digit map letters is OPTIONAL.  A gateway receiving a digit
   map with an extension digit map letter not supported SHOULD return
   error code 537 (unknown digit map extension).

   A digit map, according to this syntax, is defined either by a (case
   insensitive) "string" or by a list of strings.  Each string in the
   list is an alternative numbering scheme, specified either as a set of
   digits or timers, or as an expression over which the gateway will
   attempt to find a shortest possible match.  The following constructs
   can be used in each numbering scheme:

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   * Digit:    A digit from "0" to "9".
   * Timer:    The symbol "T" matching a timer expiry.
   * DTMF:     A digit, a timer, or one of the symbols "A", "B", "C",
               "D", "#", or "*".  Extensions may be defined.
   * Wildcard: The symbol "x" which matches any digit ("0" to "9").
   * Range:    One or more DTMF symbols enclosed between square brackets
               ("[" and "]").
   * Subrange: Two digits separated by hyphen ("-") which matches any
               digit between and including the two.  The subrange
               construct can only be used inside a range construct,
               i.e., between "[" and "]".
   * Position: A period (".") which matches an arbitrary number,
               including zero, of occurrences of the preceding
               construct.

   A gateway that detects events to be matched against a digit map MUST
   do the following:

   1) Add the event code as a token to the end of an internal state
      variable for the endpoint called the "current dial string".

   2) Apply the current dial string to the digit map table, attempting a
      match to each expression in the digit map.

   3) If the result is under-qualified (partially matches at least one
      entry in the digit map and doesn't completely match another
      entry), do nothing further.

   If the result matches an entry, or is over-qualified (i.e., no
   further digits could possibly produce a match), send the list of
   accumulated events to the Call Agent.  A match, in this
   specification, can be either a "perfect match," exactly matching one
   of the specified alternatives, or an impossible match, which occurs
   when the dial string does not match any of the alternatives.
   Unexpected timers, for example, can cause "impossible matches".  Both
   perfect matches and impossible matches trigger notification of the
   accumulated digits (which may include other events - see Section
   2.3.3).

   The following example illustrates the above.  Assume we have the
   digit map:

      (xxxxxxx|x11)

   and a current dial string of "41".  Given the input "1" the current
   dial string becomes "411".  We have a partial match with "xxxxxxx",
   but a complete match with "x11", and hence we send "411" to the Call
   Agent.

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   The following digit map example is more subtle:

     (0[12].|00|1[12].1|2x.#)

   Given the input "0", a match will occur immediately since position
   (".") allows for zero occurrences of the preceding construct.  The
   input "00" can thus never be produced in this digit map.

   Given the input "1", only a partial match exists.  The input "12" is
   also only a partial match, however both "11" and "121" are a match.

   Given the input "2", a partial match exists.  A partial match also
   exists for the input "23", "234", "2345", etc.  A full match does not
   occur here until a "#" is generated, e.g., "2345#".  The input "2#"
   would also have been a match.

   Note that digit maps simply define a way of matching sequences of
   event codes against a grammar.  Although digit maps as defined here
   are for DTMF input, extension packages can also be defined so that
   digit maps can be used for other types of input represented by event
   codes that adhere to the digit map syntax already defined for these
   event codes (e.g., "1" or "T").  Where such usage is envisioned, the
   definition of the particular event(s) SHOULD explicitly state that in
   the package definition.

   Since digit maps are not bounded in size, it is RECOMMENDED that
   gateways support digit maps up to at least 2048 bytes per endpoint.

2.1.6 Packages

   MGCP is a modular and extensible protocol, however with extensibility
   comes the need to manage, identify, and name the individual
   extensions.  This is achieved by the concept of packages, which are
   simply well-defined groupings of extensions.  For example, one
   package may support a certain group of events and signals, e.g.,
   off-hook and ringing, for analog access lines.  Another package may
   support another group of events and signals for analog access lines
   or for another type of endpoint such as video.  One or more packages
   may be supported by a given endpoint.

   MGCP allows the following types of extensions to be defined in a
   package:

   * BearerInformation

   * LocalConnectionOptions

   * ExtensionParameters

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   * ConnectionModes

   * Events

   * Signals

   * Actions

   * DigitMapLetters

   * ConnectionParameters

   * RestartMethods

   * ReasonCodes

   * Return codes

   each of which will be explained in more detail below.  The rules for
   defining each of these extensions in a package are described in
   Section 6, and the encoding and syntax are defined in Section 3 and
   Appendix A.

   With the exception of DigitMapLetters, a package defines a separate
   name space for each type of extension by adding the package name as a
   prefix to the extension, i.e.:

      package-name/extension

   Thus the package-name is followed by a slash ("/") and the name of
   the extension.

   An endpoint supporting one or more packages may define one of those
   packages as the default package for the endpoint.  Use of the package
   name for events and signals in the default package for an endpoint is
   OPTIONAL, however it is RECOMMENDED to always include the package
   name.  All other extensions, except DigitMapLetter, defined in the
   package MUST include the package-name when referring to the
   extension.

   Package names are case insensitive strings of letters, hyphens and
   digits, with the restriction that hyphens shall never be the first or
   last character in a name.  Examples of package names are "D", "T",
   and "XYZ".  Package names are not case sensitive - names such as
   "XYZ", "xyz", and "xYz" are equal.

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   Package definitions will be provided in other documents and with
   package names and extensions names registered with IANA.  For more
   details, refer to section 6.

   Implementers can gain experience by using experimental packages.  The
   name of an experimental package MUST start with the two characters
   "x-"; the IANA SHALL NOT register package names that start with these
   characters, or the characters "x+", which are reserved.  A gateway
   that receives a command referring to an unsupported package MUST
   return an error (error code 518 - unsupported package, is
   RECOMMENDED).

2.1.7 Events and Signals

   The concept of events and signals is central to MGCP.  A Call Agent
   may ask to be notified about certain events occurring in an endpoint
   (e.g., off-hook events) by including the name of the event in a
   RequestedEvents parameter (in a NotificationRequest command - see
   Section 2.3.3).

   A Call Agent may also request certain signals to be applied to an
   endpoint (e.g., dial-tone) by supplying the name of the event in a
   SignalRequests parameter.

   Events and signals are grouped in packages, within which they share
   the same name space which we will refer to as event names in the
   following.  Event names are case insensitive strings of letters,
   hyphens and digits, with the restriction that hyphens SHALL NOT be
   the first or last character in a name.  Some event codes may need to
   be parameterized with additional data, which is accomplished by
   adding the parameters between a set of parentheses.  Event names are
   not case sensitive - values such as "hu", "Hu", "HU" or "hU" are
   equal.

   Examples of event names can be "hu" (off hook or "hang-up"
   transition), "hf" (hook-flash) or "0" (the digit zero).

   The package name is OPTIONAL for events in the default package for an
   endpoint, however it is RECOMMENDED to always include the package
   name.  If the package name is excluded from the event name, the
   default package name for that endpoint MUST be assumed.  For example,
   for an analog access line which has the line package ("L") as a
   default with dial-tone ("dl") as one of the events in that package,
   the following two event names are equal:

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      L/dl

   and

      dl

   For any other non-default packages that are associated with that
   endpoint, (such as the generic package for an analog access
   endpoint-type for example), the package name MUST be included with
   the event name.  Again, unconditional inclusion of the package name
   is RECOMMENDED.

   Digits, or letters, are supported in some packages, notably "DTMF".
   Digits and letters are defined by the rules "Digit" and "Letter" in
   the definition of digit maps.  This definition refers to the digits
   (0 to 9), to the asterisk or star ("*") and orthotrope, number or
   pound sign ("#"), and to the letters "A", "B", "C" and "D", as well
   as the timer indication "T".  These letters can be combined in "digit
   string" that represents the keys that a user punched on a dial.  In
   addition, the letter "X" can be used to represent all digits (0 to
   9).  Also, extensions MAY define use of other letters.  The need to
   easily express the digit strings in earlier versions of the protocol
   has a consequence on the form of event names:

   An event name that does not denote a digit MUST always contain at
   least one character that is neither a digit, nor one of the letters
   A, B, C, D, T or X (such names also MUST NOT just contain the special
   signs "*", or "#").  Event names consisting of more than one
   character however may use any of the above.

   A Call Agent may often have to ask a gateway to detect a group of
   events.  Two conventions can be used to denote such groups:

   * The "*" and "all" wildcard conventions (see below) can be used to
     detect any event belonging to a package, or a given event in many
     packages, or any event in any package supported by the gateway.

   * The regular expression Range notation can be used to detect a range
     of digits.

   The star sign (*) can be used as a wildcard instead of a package
   name, and the keyword "all" can be used as a wildcard instead of an
   event name:

   * A name such as "foo/all" denotes all events in package "foo".

   * A name such as "*/bar" denotes the event "bar" in any package
     supported by the gateway.

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   * The name "*/all" denotes all events supported by the endpoint.

   This specification purposely does not define any additional detail
   for the "all packages" and "all events" wildcards.  They provide
   limited benefits, but introduce significant complexity along with the
   potential for errors.  Their use is consequently strongly
   discouraged.

   The Call Agent can ask a gateway to detect a set of digits or letters
   either by individually describing those letters, or by using the
   "range" notation defined in the syntax of digit strings.  For
   example, the Call Agent can:

   * Use the letter "x" to denote" digits from 0 to 9.
   * Use the notation "[0-9#]" to denote the digits 0 to 9 and the pound
     sign.

   The individual event codes are still defined in a package though
   (e.g., the "DTMF" package).

   Events can by default only be generated and detected on endpoints,
   however events can be also be defined so they can be generated or
   detected on connections rather than on the endpoint itself (see
   Section 6.6).  For example, gateways may be asked to provide a
   ringback tone on a connection.  When an event is to be applied on a
   connection, the name of the connection MUST be added to the name of
   the event, using an "at" sign (@) as a delimiter, as in:

      G/rt@0A3F58

   where "G" is the name of the package and "rt" is the name of the
   event.  Should the connection be deleted while an event or signal is
   being detected or applied on it, that particular event detection or
   signal generation simply stops.  Depending on the signal, this may
   generate a failure (see below).

   The wildcard character "*" (star) can be used to denote "all
   connections".  When this convention is used, the gateway will
   generate or detect the event on all the connections that are
   connected to the endpoint.  This applies to existing as well as
   future connections created on the endpoint.  An example of this
   convention could be:

      R/qa@*

   where "R" is the name of the package and "qa" is the name of the
   event.

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   When processing a command using the "all connections" wildcard, the
   "*" wildcard character applies to all current and future connections
   on the endpoint, however it will not be expanded.  If a subsequent
   command either explicitly (e.g., by auditing) or implicitly (e.g., by
   persistence) refers to such an event, the "*" value will be used.
   However, when the event is actually observed, that particular
   occurrence of the event will include the name of the specific
   connection it occurred on.

   The wildcard character "$" can be used to denote "the current
   connection".  It can only be used by the Call Agent, when the event
   notification request is "encapsulated" within a connection creation
   or modification command.  When this convention is used, the gateway
   will generate or detect the event on the connection that is currently
   being created or modified.  An example of this convention is:

      G/rt@$

   When processing a command using the "current connection" wildcard,
   the "$" wildcard character will be expanded to the value of the
   current connection.  If a subsequent command either explicitly (e.g.,
   by auditing) or implicitly (e.g., by persistence) refers to such an
   event, the expanded value will be used.  In other words, the "current
   connection" wildcard is expanded once, which is at the initial
   processing of the command in which it was explicitly included.

   The connection id, or a wildcard replacement, can be used in
   conjunction with the "all packages" and "all events" conventions. For
   example, the notation:

      */all@*

   can be used to designate all events on all current and future
   connections on the endpoint.  However, as mentioned before, the use
   of the "all packages" and "all events" wildcards are strongly
   discouraged.

   Signals are divided into different types depending on their behavior:

   * On/off (OO):  Once applied, these signals last until they are
     turned off.  This can only happen as the result of a reboot/restart
     or a new SignalRequests where the signal is explicitly turned off
     (see later).  Signals of type OO are defined to be idempotent, thus
     multiple requests to turn a given OO signal on (or off) are

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     perfectly valid and MUST NOT result in any errors.  An On/Off
     signal could be a visual message-waiting indicator (VMWI).  Once
     turned on, it MUST NOT be turned off until explicitly instructed to
     by the Call Agent, or as a result of an endpoint restart, i.e.,
     these signals will not turn off as a result of the detection of a
     requested event.

   * Time-out (TO):  Once applied, these signals last until they are
     either cancelled (by the occurrence of an event or by not being
     included in a subsequent (possibly empty) list of signals), or a
     signal-specific period of time has elapsed.  A TO signal that times
     out will generate an "operation complete" event.  A TO signal could
     be "ringback" timing out after 180 seconds.  If an event occurs
     prior to the 180 seconds, the signal will, by default, be stopped
     (the "Keep signals active" action - see Section 2.3.3 - will
     override this behavior).  If the signal is not stopped, the signal
     will time out, stop and generate an "operation complete" event,
     about which the Call Agent may or may not have requested to be
     notified.  If the Call Agent has asked for the "operation complete"
     event to be notified, the "operation complete" event sent to the
     Call Agent SHALL include the name(s) of the signal(s) that timed
     out (note that if parameters were passed to the signal, the
     parameters will not be reported).  If the signal was generated on a
     connection, the name of the connection SHALL be included as
     described above.  Time-out signals have a default time-out value
     defined for them, which MAY be altered by the provisioning process.
     Also, the time-out period may be provided as a parameter to the
     signal (see Section 3.2.2.4).  A value of zero indicates that the
     time-out period is infinite.  A TO signal that fails after being
     started, but before having generated an "operation complete" event
     will generate an "operation failure" event which will include the
     name of the signal that failed.  Deletion of a connection with an
     active TO signal will result in such a failure.

   * Brief (BR):  The duration of these signals is normally so short
     that they stop on their own.  If a signal stopping event occurs, or
     a new SignalRequests is applied, a currently active BR signal will
     not stop.  However, any pending BR signals not yet applied MUST be
     cancelled (a BR signal becomes pending if a NotificationRequest
     includes a BR signal, and there is already an active BR signal). As
     an example, a brief tone could be a DTMF digit. If the DTMF digit
     "1" is currently being played, and a signal stopping event occurs,
     the "1" would play to completion.  If a request to play DTMF digit
     "2" arrives before DTMF digit "1" finishes playing, DTMF digit "2"
     would become pending.

   Signal(s) generated on a connection MUST include the name of that
   connection.


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